In the viewport I see the back part of the model instead of the front
[FIX as of jan 2021] after having this problem again I was able to pinpoint the Problem to be only in orthogrraphic view, the fix for this was then changing the clip end, I've also read that changing the clip start can also fix this
[FIX as of 2020] I have no clue why but here's how I fixed this.

Go into Material preview (press z in the viewport)
Press the Arrow in the top right corner and activated these two settings



Deactivate them again and it worked for me. The problem was probably just that the viewport had an error and just needed to update the lighting, which is what this did. Hope this helps anyone in the future